The introduction of a keel or centreboard on a marine vessel such as a boat counteracted the problem of unwanted leeward motion of the vessel and also assisted in its stability. Other types of fin fitted to other types of waterborne craft tend to serve similar purposes but in addition, may also be targeted towards solving or counteracting other problems that the craft experiences in the water.
For example, the design of a surfboard is ultimately a compromise between speed and maneuverability as a surfer moves through the surf. Typically, large fins tend to provide greater control but at a cost of increased drag and as a consequence reducing the top speed of the board. Increasing competition and professionalism in the sport of surfing (be it windsurfing or traditional wave surfing) has led to a desire for improved equipment performance.
As such, the traditional long board with a fixed large “keel” fin located towards one end along the longitudinal axis of the board has to all intents and purposes, been replaced by a shorter surfboard and a fixed three fin configuration.
The same thing, albeit to a lesser extent has occurred for sail boards. In either case, in this contemporary configuration, the keel fin has become smaller with two ‘mirror image’, cambered lateral fins being located on either side of the board's longitudinal axis and near to the edge of the board.
Shaped like an aircraft wing in cross section with a high pressure substantially planar surface on one side and a low pressure curved surface on the other, these lateral or side fins once fitted to the board, point towards the longitudinal axis of the board and are substantially equidistant from the keel fin.
With such an arrangement, a surfer who has ‘caught a wave’ and is in the process of propelling the board through the water in a straight line, feels the effects of the physics of fluid mechanics which dictate inter alia that so called ‘attached laminar fluid flow’ around either side of each of the fins occurs following the Coanda effect during this linear motion.
During high turns rates however, the fins can eventually experience an interruption of this laminar flow as the fluid stream is no longer able to follow the fin's surface and turbulence begins to occur typically initially along the low pressure surface of a side fin. This turbulence and onset of the loss of the Coanda effect is shown in FIG. 1 which is a stylised view of the underside of a conventional three fin surfboard and water flow across one of the lateral fins.
Once turbulence occurs the benefit of the differential pressure on either side of a fin begins to be lost. In other words, control of the board as it cuts through the water begins to be lost.
It has been said amongst the surfing community that continued turning of the surfboard, especially at high turn rates leads to the start of a well known phenomenon called cavitation, that is the sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces (for example those resulting from the rotation of a marine propeller).
This so-called cavitation can cause a surfer travelling at very high speed to experience a drastic ‘spin out’ as the attached water flow passing over the low pressure side of the fin ‘detaches’ leading to a total loss of control of the board and an otherwise ungainly entry into the water for the surfer.
These days, there is a move to remove the word “cavitation” from the vocabulary of surfers as the phenomenon that causes ‘spin out’ is probably not cavitation (as the speeds required are unlikely to occur even in the most extreme of surfing conditions) but more probably a phenomenon known as ventilation.
Ventilation occurs when a fin is out of the water momentarily (which often occurs during surfing and especially on turning) and a large a amount of surface air is violently ‘sucked’ down the low pressure side of the fin in substantially the direction of the longitudinal axis of the fin, interrupting attached water flow causing it to ‘detach’ and as a consequence, causing spin out to occur.
Attempts to overcome this problem of spin out caused through either cavitation or ventilation has centred around the design of the lateral fins. It has been found that introducing lateral fins that are able to pivot completely, allows sharper turns to be possible but at the cost of the forward drive of the board as any movement of the fins causes a reduction in speed.
The introduction of yet smaller lateral fins increases forward drive but decrease a surfer's ability to control a turn.
This problem of compromise has existed since the fin was introduced to a board.